Recent technological advances in the increasing miniaturization of electronic circuits is made possible by advances in semiconductor processing. Certain advanced processing techniques require exposing a semiconductor's structure to a reactant gas under carefully controlled conditions of elevated temperatures, sub-ambient pressures, and uniform reactant gas flow. The examples of such processes include low-pressure CVD, reduced-pressure CVD, and selective epitaxial deposition.
Cold wall CVD systems have been used in recent years for the deposition of various semiconductor materials on silicon wafers. One of such low-pressure CVD system is a single-wafer cold-wall CVD system utilizing a high-throughput CVD chamber. More stringent standards of film quality and increasing wafer sizes utilized in recent years are diverting the equipment technology away from very large batch systems toward single-wafer processing. Such single-wafer processing equipment are being designed as a multichamber clustered integrated processing system incorporating the use of load-lock systems wherein a wafer can be transported from one single-wafer process chamber to another through a central load-lock system without breaking vacuum.
A typical single-wafer cold-wall CVD chamber is radiantly heated to enable precise temperature control in the range from about 500.degree. C. to about 1,000.degree. C. The wafer is mounted on a susceptor, which is a silicon carbide coated graphite disc, to receive uniform deposition of materials on the wafer surface. The susceptor may be rotated by a motor during deposition to further improve the uniformity of the coating. This type of thermal reactor for semiconductor processing provides for high-throughput, flexible thermal control, and process uniformity for large wafers at ambient and reduced pressures.
Reactant gases enter the CVD chamber and produce films of various electronic materials on the surface of a wafer for various purposes such as for metalization layers, for dielectric layers, for insulation layers, etc. The various electronic materials deposited include spitaxial silicon, polysilicon, silicon nitride, silicon oxide, and refractory metals such as titanium, tungsten and their silicides. In these film deposition processes, most of the material from the reactive gases is deposited on the wafer surface. However, it is inevitable that a small amount of the material is deposited on heated surfaces inside the chamber other than that of the wafer. This also occurs in a cold-wall CVD system in which the chamber wall is kept cool by the circulation of cooling air outside the chamber to avoid deposition of materials on the wall.
It is therefore necessary after a certain number of deposition processes to clean the heated surface inside the chamber of deposited materials. These heated surfaces include the exposed surface of the susceptor, the surface of the preheat ring, and any other heated surfaces inside the chamber.
Various in-situ cleaning methods have been developed for cleaning the chamber surface of a CVD system. These in-situ cleaning methods present an improvement over the conventional wet chemical cleaning method in which the components need to be disassembled and cleaned in strong acids resulting in great expenses of labor and downtime.
Various in-situ chamber cleaning methods have been proposed by others. For instance, conventional silicon and polysilicon processes use hydrogen chloride gas (HCl) at high temperatures, i.e., in the range between 1,100.degree. C. to 1,200.degree. C., for chamber cleaning. Others have cleaned plasma chamber at low temperatures, i.e., less than 400.degree. C. with various chemistries. One of the cleaning gases used in these cleaning methods has been nitrogen trifluoride (NF.sub.3).
Nitrogen trifluoride when used at high temperatures or with a plasma is extremely aggressive and etches almost all materials in the chamber including quartz, silicon carbide, and all metals. Nitrogen trifluoride has also been used in hot-wall CVD systems without a plasma. However, the quartz chamber walls can be seriously etched and any moisture will significantly change the etching characteristics due to the formation of the extremely reactive hydrogen fluoride (HF).
A prior-art method of using nitrogen trifluoride as an in-situ cleaning agent for cleaning CVD hardware is disclosed in published UK Patent Application GB2183204A. Nitrogen trifluoride was introduced into a heated CVD reactor under a partial pressure for a period of time sufficient to clean the deposited films of material. The temperature of the heated chamber for cleaning was between 380.degree. C. and 500.degree. C. and the pressure of pure nitrogen trifluoride gas used was between 200 and 600 Torr. The publication suggested that, since nitrogen trifluoride is highly toxic and explosive under pressure, it is desirable to use lower pressures of nitrogen trifluoride for health, safety and environmental reasons even though only unacceptably low etch rates can be realized at such low chamber pressures. It is noted that the temperature range disclosed by the publication is very flow, i.e. 380.degree. C..about.500.degree. C. This is necessitated by the fact that moisture was present in the chamber which contributed to the formation of hydrogen fluoride. The lower temperature range was necessary to contain the reactivity of hydrogen fluoride in order to reduce the etching damage to the chamber hardware.
It is therefore an object of the present invention to provide a method of in-situ cleaning a cold-wall CVD chamber by a reactive gas that does not have the shortcomings of conventional wet chemical cleaning methods.
It is another object of the present invention to provide a method of in-situ cleaning a cold-wall CVD chamber with a reactive gas at a sufficiently low pressure while maintaining a satisfactory etch rate.
It is a further object of the present invention to provide a method of in-situ cleaning a cold-wall CVD chamber with a reactive gas by providing a substantially moisture-free chamber environment such that a reactive gas at a sufficiently low pressure may be used while maintaining a satisfactory etch rate.
It is another further object of the present invention to provide an etchant gas system for in-situ cleaning of a cold-wall CVD chamber which can be used at low chamber pressures and low chamber temperatures in a substantially moisture-free environment to achieve a high etch rate.